Multichannel echo canceller system using active audio matrix coefficients

ABSTRACT

An echo canceller system is described, which comprises an audio matrix decoder coupled to audio inputs for providing multiple audio output signals, and a multi-channel echo canceller coupled to the audio inputs. The echo canceller system further comprises audio receiving means for receiving a mixture of the audio output signals and a wanted signal. The multi-channel echo canceller is coupled to the audio matrix decoder for using the audio matrix coefficients with multi-channel echo cancellation for deriving the wanted signal from the mixture. The computational and audio processing complexity of such a system is reduced.

The present invention relates to an echo canceller system, comprising anaudio matrix decoder coupled to audio inputs for providing multipleaudio output signals, and a multi-channel echo canceller coupled to theaudio inputs.

The present invention also relates to a multi-channel audio system, forexample a Hi-Fi set, a car audio system or a television orteleconferencing system and to a communication system, such as a mobileor hands free communication system, in particular a telephone system, orcar telephone system, which aforementioned systems may be voicecontrolled and comprise such an echo canceller. The invention alsorelates to signals for use in the echo canceller.

Such an echo canceller system is known from WO 98/42162. The known echocanceller system comprises an audio matrix decoder, such as an activeaudio surround decoder providing multiple audio surround output signalsin a multi-channel arrangement having for example three or fiveloudspeakers. The loudspeakers may e.g. be positioned in a home theatre,a personal computer “multimedia” environment and/or combined with atelevision set. The effects of echoes may be included in a transferfunction matrix acting as a multi-channel echo canceller. The known echocanceller system does however not disclose an efficient way of achievingmulti-channel echo cancelling.

It is an object of the present invention to provide an improved echocanceller system posing a reduced audio signal processing burden onmulti-channel echo cancelling, and which may also be equipped with voicecontrol features.

Thereto the echo canceller system according to the invention ischaracterized in that the echo canceller system further comprises audioreceiving means for receiving a mixture of the audio output signals anda wanted signal, and that the multi-channel echo canceller is coupled tothe audio matrix decoder for using the audio matrix coefficients withmulti-channel echo cancellation for deriving the wanted signal from themixture.

It is an advantage of the echo canceller system according to the presentinvention that by using the audio matrix coefficients present in theaudio matrix decoder or derivable possibly only from its decoder outputsignals, multi-channel echo cancelling can be effected very effectively,without high computational and/or signal processing complexity.

In addition voice control capabilities are improved, because the echocanceller system according to the invention is capable of deriving awanted signal, such as a voice signal from the mixture of sound, echoesand voice present in a room. This extends the cost effective applicationpossibilities of the echo system according to the invention in more andmore accepted voice controlled systems, such as multi-channelcommunication, audio and/or television systems, Hi-Fi sets, car audiosystems and teleconferencing systems.

An embodiment of the echo canceller system according to the invention ischaracterized in that the echo canceller system comprises an activematrix tracker coupled to both the audio inputs and matrix decoderoutputs for deriving the audio matrix coefficients therefrom.

In general the active audio matrix coefficients are only available onchip in the audio matrix decoder and not externally available, inparticular not in audio systems which are already on the market orinstalled at a customer premises. The active matrix tracker provides aneasy to apply alternative to a full exchange of such a chip, because itsimply derives the matrix coefficients from the audio input and outputsignals of the matrix decoder, which signals are already externallyavailable. Thus the active matrix tracker can easily be added to knownecho canceller systems.

A further embodiment of the echo canceller system according to theinvention is characterized in that the multi-channel echo cancellercalculates a number of filter functions which is smaller than the numberof the multiple audio output signals.

Advantageously no separate echo cancelling calculations are needed forall multiple audio output signals, which reduces the computationalcomplexity of the multi-channel echo cancelling even further.

A still further embodiment of the echo canceller system according to theinvention is characterized in that the filter functions calculations arebased on filter update contributions which take account of at least someof the audio output signals.

Despite the reduction of the computational complexity of the audiocancelling filter function calculation, the quality of the echocancelled output signals is not effected, because the filter updatestake account of some and possibly all of the multiple audio outputsignals.

Another embodiment of the echo canceller system according to theinvention is characterized in that the echo canceller comprises a speechdetector coupled to reducing the number of independent multiple outputsignals once speech is detected.

It is an advantage that if speech is detected by the speech detectorecho cancelling can be simplified at the expense of only a small loss ofaudio quality, which will however not be noticeable because a listeneris then giving commands by his speech. At present the echo cancellersystem according to the invention will be elucidated further togetherwith its additional advantages, while reference is being made to theappended drawing, wherein similar components are being referred to bymeans of the same reference numerals.

In the drawings:

FIG. 1 shows a schematic view of combined possible embodiments of theecho canceller system according to the invention; and

FIG. 2 shows a detailed embodiment of the echo canceller for applicationin the echo canceller system of FIG. 1.

FIG. 1 shows an echo canceller system 1, comprising an audio matrixdecoder 2, for example a Dolby Pro Logic I/II, Circle Surround system,or the like. The audio matrix decoder 2 as shown is coupled to audioinputs 3 carrying two channels, in this case left and right stereosignals x′_(L) and x′_(R) respectively. Here five multiple audio signalsx₁, x₂, x₃, x₄ and x₅ are derived from the stereo signals by the decoder2 and supplied to five loudspeakers generally designated 4, in a room R.The system 1 also comprises a multi-channel echo canceller 5 coupled tothe audio inputs 3. The echo canceller 5 provides an echo cancellingsignal y at its output 6. Audio receiving means 7 generally embodied byone or more microphones are placed in the room R for receiving a mixtureof the five audio output signals and a wanted speech signal originatingfrom a listener/speaker S in the room R. A subtracter 8 is coupled toboth the microphone 7 and the echo canceller output 6 for providing anaccurate estimate of the wanted speech signal e. Such a speech signal emay be used and included in any system for providing voice controllingcommands. Examples of voice control systems possibly implementing thosecommands are a Hi-Fi set, a car audio system, a television. Examples ofcommunications systems are a teleconferencing system, a mobile or handsfree communication system, in particular a telephone system, cartelephone system and the like.

The relation between input signals and output signals of the matrixdecoder 2 can be written in vector notation as:X=A (α_(i); β_(i))X′  (1)where the vector X represents the five audio signals x_(i) with i=1 . .. 5, the vector X′ represents the stereo input signals x_(j) with j=L, Rand A(α_(i); β_(i)) represents a 2×5 coefficient matrix having tencoefficients. The multi-channel echo canceller 5 is coupled to the audiomatrix decoder 2, where these coefficients are used by the echocanceller in a manner to be explained hereafter for deriving the wantedsignal from the mixture of signals received by the receiving microphone7.

In an alternative embodiment also shown in FIG. 1 the echo cancellersystem 1 comprises an active matrix tracker 9 coupled to both the audioinputs 3 and matrix decoder outputs 10 for deriving the ten audio matrixcoefficients of the matrix A(α_(i); β_(i)) (having 5 coefficients α_(i)in its first column and 5 coefficients β_(i) in its second column)therefrom, in accordance with equation (1). The tracker 9 may beincluded in the matrix decoder 2 or simply added later thereto. It isalso possible that the tracker 9 is only coupled to the decoder output10 (and thus indirectly coupled to its input), which tracker 9 is thencapable of deriving the decoder coefficients from the decoder outputsignals alone. This is advantageous in case the decoder inputs 3 and/orthe matrix coefficients are not—easily externally—available. In thealternative embodiment mentioned before the active matrix tracker 9 iscoupled to the echo canceller 5 where again these coefficients are usedby the echo canceller for deriving the wanted signal from the mixture ofsignals received by the receiving microphone 7.

A straightforward approach to the problem of cancelling the echoesarising between the five loudspeakers 4 and the microphone 7 would be tocouple the five decoder outputs 10 to the echo canceller 5. Thecanceller 5 then would have to implement a five channel adaptive filterrequiring a high computational complexity. Apart therefrom additionalproblems arise involving the uniqueness of the solutions which theadaptive filters are capable of finding. FIG. 1 shows that the echocanceller 5 is directly coupled to the audio input 3, and that use ismade of the ten active matrix coefficients (α_(i); β_(i)) made availablefor echo cancelling.

As suggested by the representation of FIG. 2 the multi-channel echocanceller 5 calculates a number of filter transfer functions with isless than the number of the multiple audio output signals. The fivetransfer functions between the loudspeakers 4 and the microphone 7 mayin particular be represented by only two filter transfer functions w_(L)and w_(R). This reduces the computational complexity of the echocanceller 5. These transfer functions may be updated by updates Δ_(L)and Δ_(R) such that the added echo canceller output signal y representsthe microphone signal exclusive the wanted speech signal as accurate aspossible, according to:w _(L) =w′ _(L)+Δ_(L)w _(R) =w′ _(R)+Δ_(R)where the ′ denoted the previous value. Bookkeeping may involve thedistribution of the updates over all five transfer filters, which may beaccording to:w _(i) =w′ _(i)+α_(i)(/Σ|α_(i)|)Δ_(L)+β_(i)(/Σ|β_(i)|)Δ_(R) for i=1, 2,. . . 5.Where the sigma represents a summation over i, and where α_(i) and β_(i)are the coefficients of the matrix A. The relation between the filtersis now given by:w _(L)=Σ(α_(i) w _(i))w _(R)=Σ(β_(i) w _(i))In this case all five acoustical paths are tracked, which isadvantageous in case the active matrix changes in time. When convergenceis achieved, the active matrix may suddenly change and the correctfilters are obtained immediately by latter equations.

In some applications for example involving Pro-Logic, or Pro-Logic 2decoders some of the five filters may be joined to reduce the burden ofthe necessary amount of calculations. When mono surround is used forexample, both acoustical paths associated with surround are trackedusing only one filter. In case of possible singularities or numericaloverflows in the echo cancelling calculations the five channels may bemapped into lesser channels in order to resolve these kinds of errors.It is also possible to down mix multi-channel audio to stereo whenspeech activity is detected by a speech detector 11 to be coupled to theecho canceller 5. This advantageously allows the application of a muchsimpler echo canceller at only a small temporarily loss of soundquality.

Stated simple the audio matrix decoder 2 may derive a number of outputsignals from its input, which number exceeds the number of inputsignals. The decoder 2 may comprise one or more filters, for examplecontrollable filters, whose filter operation and filter function dependson control signals. These control signals may for example be included inthe music.

Whilst the above has been described with reference to essentiallypreferred embodiments and best possible modes it will be understood thatthese embodiments are by no means to be construed as limiting examplesof the system concerned, because various modifications, features andcombinations of features falling within the scope of the appended claimsare now within reach of the skilled person.

1. Echo canceller system (1), comprising an audio matrix decoder (2)coupled to audio inputs (3) for providing multiple audio output signals(x₁ . . . x₅), and a multi-channel echo canceller (5) coupled to theaudio inputs (3), characterized in that the echo canceller system (1)further comprises audio receiving means (7) for receiving a mixture ofthe audio output signals (x₁ . . . x₅) and a wanted signal (e), and thatthe multi-channel echo canceller (5) is coupled to the audio matrixdecoder (2) for using the audio matrix coefficients with multi-channelecho cancellation for deriving the wanted signal (e) from the mixture.2. Echo canceller system (1) according to claim 1, characterized in thatthe echo canceller system (1) comprises an active matrix tracker (9)coupled to both the audio inputs (3) and matrix decoder outputs (10) forderiving the audio matrix coefficients therefrom.
 3. Echo cancellersystem (1) according to claim 1, characterized in that the multi-channelecho canceller (5) calculates a number of filter functions with is lessthan the number of the multiple audio output signals (x₁ . . . x₅). 4.Echo canceller system (1) according to claim 3, characterized in thatthe filter functions calculations are based on filter updatecontributions which take account of at least some of the audio outputsignals (x₁ . . . x₅).
 5. Echo canceller system (1) according to claim1, characterized in that the echo canceller (5) comprises a speechdetector (11) coupled to reducing the number of independent multipleoutput signals (x₁ . . . x₅) once speech is detected.
 6. Multi-channelaudio system, for example a Hi-Fi set, a car audio system or atelevision or teleconferencing system, comprising an echo cancellersystem (1) according to claim 1, the echo canceller system (1)comprising an audio matrix decoder (2) coupled to audio inputs (3) forproviding multiple audio output signals (x₁ . . . x₅), and amulti-channel echo canceller (5) coupled to the audio inputs (3),characterized in that the echo canceller system (1) further comprisesaudio receiving means (7) for receiving a mixture of the audio outputsignals (x₁ . . . x₅) and a wanted signal (e), and that themulti-channel echo canceller (5) is coupled to the audio matrix decoder(2) for using the audio matrix coefficients with multi-channel echocancellation for deriving the wanted signal (e) from the mixture. 7.Communication system, such as a mobile or hands free communicationsystem, in particular a telephone system, car telephone system,comprising an echo canceller system (1) according to claim 1, the echocanceller system (1) comprising an audio matrix decoder (2) coupled toaudio inputs (3) for providing multiple audio output signals (x₁ . . .x₅), and a multi-channel echo canceller (5) coupled to the audio inputs(3), characterized in that the echo canceller system (1) furthercomprises audio receiving means (7) for receiving a mixture of the audiooutput signals (x₁ . . . x₅) and a wanted signal (e), and that themulti-channel echo canceller (5) is coupled to the audio matrix decoder(2) for using the audio matrix coefficients with multi-channel echocancellation for deriving the wanted signal (e) from the mixture. 8.Signals for use in the echo canceller system (1) according to claim 1,the echo canceller system (1) comprising an audio matrix decoder (2)coupled to audio inputs (3) for providing multiple audio output signals(x₁ . . . x₅), and a multi-channel echo canceller (5) coupled to theaudio inputs (3), characterized in that the echo canceller system (1)further comprises audio receiving means (7) for receiving a mixture ofthe audio output signals (x₁ . . . x₅) and a wanted signal (e), and thatthe multi-channel echo canceller (5) is coupled to the audio matrixdecoder (2) for using the audio matrix coefficients with multi-channelecho cancellation for deriving the wanted signal (e) from the mixture.